Three patterns of hematogenous pyelonephritis occurred which represent an interrelationship between an immune response to the infecting bacteria and the development of obstructive uropathy as a consequence of infection. First, the course of pyelonephritis due to strains of Escherichia coli was acute, self-limited, and associated with the development of circulating agglutinins. Following healing, this pattern of infection was associated with acquired resistance to reinfection with the same bacterial strain. Second, the course of pyelonephritis due to a strain of Klebsiella pneumoniae type C was chronic. This infection was not associated with the production of circulating agglutinins against encapsulated strains. Acquired resistance to reinfection by the homotypic strain could not be demonstrated following eradication of infection but was produced by the passive transfer of concentrated antiserum. Third, pyelonephritis due to Proteus mirabilis was associated with circulating agglutinins and resistance to reinfection with the same organism following eradication of infection, yet the course was chronic. The chronicity appeared to be the consequence of obstructive uropathy resulting from calculi which developed during the course of the infection.

Resistance to reinfection was demonstrated in infections with strains of E. coli and P. mirabilis. The resistance is associated with specific immunity as demonstrated by the observation that: (a) it is type-specific, (b) it is of at least 3 months' duration, and (c) it can be passively transferred by means of rabbit antiserum. Since K. pneumoniae failed to evoke capsular antibodies in the rat, resistance to infection with K. pneumoniae was produced only by means of passively transferred concentrated rabbit antiserum and not by prior infection.

Immunity can be demonstrated to have a significant role in the pathogenesis of experimental hematogenous pyelonephritis only in the absence of obstructive uropathy.